Electric heating device



March 15, 49 v. R. W. Boa-@EINER Zl ELECTRIC HEATING DEVICE Filed Aug. l5, 1946 A TTOIQNEY Patented Mar. 15, 1949 ELECTRIC HEATING DEVICE Victor Rudolph William Bohener, London, England, assignor to William Edwin Smith, Elstree, Hertfordshire, and William Frost, London, England Application August 15, 1946, Serial No. 690,621 In Great Britain April 18, 1945 Section 1, Public Law 690, August 8, 1946. Patent expires April 18, 1965 Cl. 21S-19) 2 Claims. 1

This invention relates to control devices for electric circuits, such as heating devices and has for its object to provide a device which will automatically open the circuit after a predetermined interval of time. device it will provide a predetermined amount of heat, and if desired at a predetermined temperature, and then interrupt the heating circuit so that no more heat is produced. Such a device is particularly suitable for use in the heat treatment of materials to relieve the operator oi responsibility and of the necessity for continuous attention to the process. The vulcanising of rubber is an example of such a proce-ss, but the invention may be used for other processes such as the heat treatment of plastics.

In the accompanying drawings* Figure 1 is a purely diagrammatic representation of the improved heating device, and the mode of its use and Figure 2 is a front View partly broken away of the resistance element used in the heating device.

Referring iirst to Figure 1, this illustrates in diagrammatic form the application of this, invention to a vulcaniser. Any convenient table or platen IIJ is used to support the work which is indicated as a sheet of rubber I I wherein a patch I2 is to be vulcanised. The heating device by which the required heat is imparted to the work is constituted by a block I3 of metal or other suitable material which is engaged with the work II', I2 on the platen. Pressure may be applied to this block when used for vulcanising as is customary, although other kinds of heat treatment may not necessitate the application of pressure.

On the side of the thermal storage member I3 remote from the platen there is mounted a heating element I4 which comprises a suitable casing or cover with an electric resistance unit. On the other side of the heating element Ill remote from the storage member I3 a heat insulating block or cover I5 may be provided to prevent loss ci heat from that side of the heating element.

The heating element itself is illustrated in Figure 2 and comprises an electrical resistance constituted by a thin strip of lead foil or other suitable material which is sandwiched between two sheets of paper I1, I8 so as to provide ariequate support for it and to enable it to be readily handled. This i-s necessary since normally the resistance element I6 will be of such limited cross-section that it is mechanically weak. Suitable contacts or terminals I9 are provided for making electrical connection to the resistance I6.

When used with a heating The material of which the resistance element I6 is made is fusible in order that when it has attained a -suiiiciently high temperature it shall melt and break the electric circuit so that there is no further output oi heat from it.

In the operation of the device when the current is switched on, the rate of evolution of heat is determined by the value of the resistance which determines the current, and whilst the heat is being evolved it is simultaneously being absorbed by the thermal storage member I3. The thermal conductivity of this member is selected so that it absorbs heat at a rate which is somewhat less than, but is comparable with, the rate of evolution of heat so that the temperature-rise of the resistance element I6 is retarded. It will be appreciated that the thermal storage element I3 has to attain a temperature comparable with the melting point of the resi-stance material before the latter will melt, and the time which it takes to reach this temperature is determined mainly by its heat-capacity, and also by its conductivity; the conductivity is a controlling factor from two points of View in so far as it determines the rate at which the storage member will absorb heat, and also the rate at which it will transmit heat to the Work which is on the other side of the storage member from the heating element. 'I'hese two factors therefore determine the time which must elapse before the resistance element I6 reaches its melting point and thereby interrupts the supply of heat.

After the supply of heat has been cut oil, the heat stored in the thermal storage member I3 is gradually transmitted to the work to effect the desired heat treatment thereof.

It will be seen that this invention provides a heating device in which the quantity of heat imparted to the work is automatically controlled in a very simple manner, and the maximum temperature to which the work can be subjected is also under control, since it cannot exceed the melting point of the electric resistance element, and will usually be somewhat lower.

A further control is available in respect of the time which elapses before the circuit is broken, which can be obtained by taking into consideration the temperature-coefcient of resistance of the material of the heating element; if a material is selected which has a very high temperature-coefficient of resistance, its resistance will increase substantially with rise of temperature so that the rate of evolution of heat falls off as the temperature rises and the time 3 which elapses before it reaches its melting point will thereby be increased.

In the heatingelement illustrated in Figure..2`, the whole of the `electric resistance material is made of the fusible metal, but it will be appreciated that this is not essential since all the functions of this element as above described can be obtained if only a part of the resistance element is fusible provided this part is subjected to and its melting determined 4by the temperature of the thermal storage element. It is preferred, however, to construct the element asshown in Figure 2 for the reasons of simplicity and minimum cost of manufacture.

Apparatus in accordance with this invention is particularly suitable for use in vulcanisingrubbei', and may be made in the form .of a portable outfit for repairing tyres or tubes of motor vehicles; the apparatus is then designed to be operated from the electric supply ordinarily available on acar, and all tha-tis necessary is to insert the heating element in the device and switch it ron. The controlaof ltheternperature and duration of heating is-automatically effected, and the resistance velement is;so inexpensive thatta new one can be used for'eachfoperation. rThe invention, howeverfis not limited to this particular purpose and it :may be applied as above mentioned to various otherheat treatments.

tI claim:

1. In apparatus l-for the .automatic heat-treatment ,of material lvata `preselected temperature,

.bination of an electric heating element in the form of va thin foil of metal fusible at the teinperature of vulcanisation, a pair of sheets of thin insulating paper enclosing said heating element, a Work-receiving platen of substantial heat-capacity in contact with said insulating paper, and a second platen adjacent said first platen to receive the -work between them.

VICTOR RUDOLPH WILLIAM BOHENER.

REFERENCES CITED The following references are of record in the le of thispatent:

UNITED STiTES PATENTS Name Date Van Dyke Mar. 30, 1937 Laurence Aug. 29,'1944 FOREIGN PATENTS Country .Date Great Britain u...K .lApin 10,'1940 Number Number 

